Semiconductor nanocrystals whose dimensions are comparable to the bulk exciton diameter show quantum confinement effects. This is seen most clearly in the optical spectra which shift towards the red and/or infrared as the size of the crystal is increased. The emission of nanocrystals is determined not only by the size of the nanocrystals, but by the uniformity of the nanocrystal sizes in a given group. Nanocrystals with a small size distribution (that is, with high uniformity of nanocrystal sizes) will provide narrow emission, while a large distribution of sizes within a nanocrystal group will provide wide emission.
Semiconductors nanocrystals made from a wide range of materials have been studied including many II-VI and III-V semiconductors. In addition to spherical nanocrystals rod-, arrow-, teardrop- and tetrapod-shaped nanocrystals [Alivisatos et. al., J. Am. Chem. Soc, 2000, 122, 12700; WO03054953] and core/shell structures [Bawendi, J. Phys. Chem. B, 1997, 1010, 9463; Li and Reiss, J. Am. Chem. Soc., 2008, 130, 11588] have also been prepared. To control the size and shape of such nanocrystals their synthesis is generally performed in the presence of one or more capping agents (sometime called surfactants or coordinating solvents). Such capping agents control the growth of the nanocrystals and also increase the intensity of the light emission though the passivation of surface states. A wide range of capping agents have been employed including phosphines [Bawendi et. al., J. Am. Chem. Soc., 1993, 115, 8706], phosphine oxides [Peng et. al., J. Am. Chem. Soc., 2002, 124, 2049], amines [Peng et. al., J. Am. Chem. Soc., 2002, 124, 2049], fatty acids [Battaglia and Peng, Nano Lett., 2002, 2, 1027; Peng et. al., J. Am. Chem. Soc., 2002, 124, 2049], thiols [Li and Reiss, J. Am. Chem. Soc., 2008, 130, 11588] and more exotic capping agents such a metal fatty acid complexes [Nann et. al., J. Mater. Chem., 2008, 18, 2653].
Methods to prepare semiconductor nanocrystals include solvothermal reactions [Gillan et. al., J. Mater. Chem., 2006, 38, 3774], hot injection methods [Battaglia and Peng, Nano Lett., 2002, 2, 1027], simple heating processes [Van Patten et. al., Chem. Mater., 2006, 18, 3915], continuous flow reactions [US2006087048] and microwave assisted synthesis [Strouse et. al., J. Am. Chem. Soc., 2005, 127, 15791]
Nitride nanocrystals have been synthesized using multiple nitrogen sources in the past; however, the nanocrystals produced have been of poor crystallinity and size quality [Gillan eta al., Chem. Mater., 2001, 13, 4290 and Wells, et al., Chem. Mater., 1998, 10, 1613]. Most importantly these nanocrystals have either exhibited no emission or very weak, broad emission, in one case a nanocrystal population had a peak width of nearly 300 nm at half the peak intensity (or full-width at half maximum, FWHM).
Emissive nitride nanocrystals have been produced before by Sharp Kabushiki Kaisha (UK patent application Nos. GB2467161 published 28 Jul. 2010, GB2482311 published 1 Feb. 2012, GB2467162 published 28 Jul. 2010). However, populations of these nanocrystals exhibited broad luminescence with FWHM of more than 110 nm.
Nanoco has previously proposed the synthesis of narrowly emissive nanocrystals through the use of molecular templates [Patents: EP1334951 published 13 Aug. 2003, U.S. Pat. No. 7,588,828 first published on 3 Jul. 2008, U.S. Pat. No. 7,803,423 first published on 30 Aug. 2008, US20070104865 published on 10 May 2007, and US20080220593 published on 11 Sep. 2008]. Nanoco propose separating the nucleation step of their precursors from the growth of their nanocrystals through the use of molecular precursor templates. They propose a process in which conversion of a precursor composition to nanoparticles is effected in the presence of a molecular cluster compound—this avoids the need for a high-temperature nucleation step to initiate nanoparticle growth because suitable nucleation sites are provided by the molecular clusters so that the molecules of the cluster compound act as a template to direct nanoparticle growth. In their synthesis, the growth of nanocrystals is initiated by the addition of at least 2 precursors to the already heated molecular cluster templates. The ions from these precursors then become part of the final nanocrystal products of the reaction, while the molecular cluster templates do not.
WO 02/053810 (published 11 Jul. 2002) proposes a method for the synthesis of quantum dot nanocrystals. It proposes that the final nanoparticle size, size distribution and yield can be controlled by introducing a reaction promoter into the reaction system. The reaction promoter is an oxygen-containing gas (ie oxygen gas itself or a gas mixture containing molecular oxygen, for example such as air).
GB 2467162 (published 28 Jul. 2010) proposes manufacturing a nitride nanostructure from constituents including: a material containing metal, silicon or boron, a material containing nitrogen, and a capping agent having an electron-accepting group for increasing the quantum yield of the nitride nanostructure. The reaction may be effected by providing all reaction constituents in a solvent and heating the reaction mixture to a desired temperature, or alternatively it may be effected by disposing some but not all constituents in the solvent, heating the mixture to the desired reaction temperature, and then introducing the remaining constituents into the heated mixture.
WO 2007/016193 (published 8 Feb. 2007) relates to a method of producing nanoparticles in which the reaction mixture is superheated, by microwaving the reaction mixture.